What class of attacks use innovative attack tools and once a system is infected it silently extracts data over an extended period?

Advanced persistent threat.

Explanation:

Advanced persistent threat is a threat actor implemented by either a government supported or private group to intrude a network or system and stays undetected, collecting information over a period of time.

It is used by cyber terrorist group to facilitate massive attacks based on the information retrieved. National or government group use the concept to promote national security.

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